1. Field of the Invention
This invention relates generally to schlieren type apparatus for monitoring turbulent flow and more particularly to focusing type schlieren apparatus.
2. Description of the Prior Art
Refraction anomalies produced in transmitted light by differences in density or other anisotropy in parcels or strata of air or other fluids are known as schlieren. Natural scintillation phenomena in the atmosphere results from density schlieren developed by turbulent processes. Schlieren apparatus is generally known and is commonly used for observing and photographing refractive index gradients which occur, for example, in the study of air flow in wind tunnels.
In its simplest configuration, apparatus for observing schlieren involves placing a pair of lens on either side of a flow field with a point light source being located at the focal point of one of the lenses. The lens adjacent the light source collimates the light emanating therefrom which is refracted as it passes through the medium of the flow field. The lens on the other side of the flow field acts as a collective lens which, if matched with the other lens, focuses the light leaving the flow field to a point and should match the size and shape of the light source. When a knife edge is placed at the second focal point in such a way to cut the light point in half, it has the effect of darkening the background from a bright white to an even gray. The knife edge then blocks light refracted in that direction showing the gradient as dark. Light refracted in the opposite direction shows up brighter against the background. The image of the turbulence in the flow field can then be viewed visually either on some type of screen or photographically on film or video.
Such schlieren systems are expensive, requiring a highly specialized light source and require a large amount of space on both sides of the flow field.
This type of apparatus has evolved into focusing schlieren systems which remove the requirement for collimated light to display density gradients and involves utilizing "sheets of light" which are generated by back lighting a high contrast grid pattern. The sheets of light are directed through the flow field onto a lens that projects the image of the grid to a focal plane where there is located a negative of the high contrast photographic grid pattern whose dark lines each act as knife edges. This element is known as a cut-off grid.
Accordingly, as the light sheets pass through the turbulence of the flow field, the sheets of light refract and are either stopped by the "knife edges" of the cut-off grid, or pass through the space therebetween and the image of the turbulence is projected through the knife edges where it can then be viewed or captured as before.
Focusing schlieren systems normally require substantial space on both sides of the test section for the lighting/grid portion and the optic/image components.
Focusing schlieren type systems are discussed in detail in a publication entitled "An Improved Large-Field Focusing Schlieren System", Leonard M. Weinstein, AIAA 91-0657, 29th Aerospace Science Meeting, Jan. 7-10, 1991, pp. 1-12. Portions of this publication comprise prior art which is recognized in the accompanying detailed description of the subject invention.